Field of the Invention
This invention relates to a truss structure composed of members of a circular cross section, and more particularly it is concerned with improvements in a joint for uniting the members to provide a truss structure.
Hitherto, in a truss structure composed of members of a circular cross section, it has been usual practice to weld the main tubes to the branch tubes directly. For example, in a two-dimensional truss structure of simple form including main tubes 1 and branch tubes 2a and 2b as shown in FIG. 1, the branch tubes 2a and 2b are united to the main tube 1 in a portion A by welding as clearly shown in FIG. 2. In the welded truss structure of this type, when a load Po is applied in the direction of an arrow in FIG. 1 to a side opposite to the side at which the structure is fixed, a load P acts on the branch tube 2a in the direction of an arrow in FIG. 2, so that local deformation develops as shown in FIG. 3 in a portion of the main tube 1 at which the branch tube 2a is connected thereto and stress of a high magnitude is produced locally.
In order to relieve such local stress, proposals have been made to use a reinforcing plate and this measure has been adopted. In this measure, a reinforcing plate 4 is welded, as shown at 6, to a portion of the main tube 1 at which the branch tubes 2a and 2b are connected thereto and then the branch tubes 2a and 2b are welded to the reinforcing plate 4 as shown at 6a and 6b respectively in FIGS. 4 and 5. By increasing the thickness of the joint of the main tube and the branch tubes, it is possible to relieve local stress. However, in a truss structure composed of members of a circular cross section, many problems are encountered besides the aforesaid problem of local stress development. They include the following:
(1) Deformation of the main tube is caused by welding. For example, as shown in FIG. 6, welding of the branch tubes 2a and 2b to the main tubes 1 causes the latter to undergo deformation as indicated by broken lines in the figure. Thus difficulties are experienced in carrying out production because high precision is required in effecting horizontal positioning and obtaining perfect straightness of the structure.
(2) Generally, a struss structure composed of members of a circular cross section has the disadvantages that operation efficiency is low, the production is time consuming and the quality of the welds is inferior to the welds produced with members of a plate form, because a plurality of branch tubes are joined to the upper and lower main tubes as shown in FIG. 6.
(3) It is necessary that in producing a truss structure of this type tolerances in dimensions should be taken into consideration so that opposite ends of the branch tubes are in intimate contact with the main tubes and no great displacements of the branch tubes occur when they are connected to the main tubes.
(4) Strength of the structure is lowered due to deformation of the members caused by welding or deterioration in the quality of the welds.
As other means for connecting the branch tubes to the main tubes, the use of a connector including pivots or pins and screws may come to mind. However, the use of such a connector makes it impossibIe to impart enough strength to the joints of the truss structure, and since there is a latitude of movement about the pivots and pins it is difficult to provide a truss structure of high strength. Thus the use of a mechanical connector for uniting the branch tubes to the main tubes is not suitable for use in a truss structure because such connecting means is required to have joint strength equal to or higher than the strength of the members constituting the truss structure.